1. Field of the Invention
The present invention relates to a control apparatus for a power conversion system including a DC/AC converter circuit connected to an electric rotating machine at its output terminals and to a DC power source at its input terminals through a switching means, a capacitor being connected across the input terminals of the DC/AC converter circuit, the control apparatus operating to control the electric rotating machine by manipulating the DC/AC converter circuit, and including a current supply means to supply a current to the electric rotating machine in order to discharge the capacitor by manipulating the DC/AC converter circuit when the switching means is set open.
2. Description of Related Art
Japanese Patent Application Laid-open No. H9-70196 describes such a control apparatus configured to control a current supplied to a three-phase permanent magnet motor such that a command current in the q-axis direction is kept at 0, and a current in the d-axis direction is kept larger than 0 in absolute value, in order to enable discharging a capacitor connected across input terminals of an inverter as a DC/AC converter circuit while preventing the motor from generating torque. This is possible because the torque T of the motor is represented by the torque formula of T=p{φ+(Ld−Lq)}id·iq, where id is a d-axis current, iq is a q-axis current, Ld is a d-axis inductance, Lq is a q-axis inductance, φ is an armature interlinkage flux constant, and p is the number of pole pairs.
The above method needs to detect an electrical angle of the motor while performing current supply control to discharge the capacitor. Accordingly, if there is an error in the detected electrical angle, the phase of the motor current (the current supplied to the motor) may be erroneously determined to be along the d-axis, as a result of which the motor generates torque. In this case, if the current supply control is performed while the motor is not stationary, the motor continues to be driven regeneratively to generate electric power and the phase of the motor current continues to be erroneously determined to be along the d-axis if this torque is a load torque. This may degrade controllability of the time necessary to complete the discharge of the capacitor.
One of factors that cause a torque in the motor is an error of the above torque formula. That is, the above torque formula is derived from a simple motor model in which the values of the d-axis inductance Ld and the q-axis inductance Lq are fixed. However, an actual motor has an inductance component resulting from spatial harmonics which are dependent on the electrical angle of the motor. Accordingly, even when the motor current is supplied such that its phase is along the d-axis, torque may occur in the motor. This disadvantage is significant in a permanent magnet motor having concentrated windings in which spatial harmonic are conspicuous.